The present invention concerns electronic watches; and more particularly electronic watches having non-moving means of control.
Means of control frequently used in electronic watch-making is an electrical contact actuated by a moving element such as a push button or a crown. This solution necessitates the use of an element which passes through the watch case, which imposes design limitations, reduces reliability and adds to the cost of fabrication of the product. On the other hand, the number of controls, and therefore the contacts and necessary actuators, is increased with the number of functions available in the watch. The problem of the control of watches having available many functions rapidly becomes critical, for obvious economic and esthetic reasons. It is hardly possible to use more than four control elements. The use of codes, which has surmounted this problem, implies the necessity to memorize an increasingly complex system of manipulation.
The introduction in watches of already well-known capacitive pick-offs or detectors used as control elements has eliminated certain problems posed by moving control elements and has allowed the conception of out of the ordinary means of entering control orders. For example, the system described in communication no. 8 delivered by J. P. JAUNIN on the occasion of the 55th Congress of the Swiss Society of Chronometry in Oct., 1980, using four capacitive pick-offs placed in line, makes possible the changing of the time or the introcution of a wake-up time in a digital watch very simple. To do this, the watch is put into correction mode using an electrical push-button, then the displayed information is modified by passing a fingertip along the centre line of the four capacitive pick-offs mounted either in the watch case or in the watch glass. The numerical value displayed is increased or decreased depending upon the direction of motion of the fingertip, the change being one unit for each passage of the fingertip across a pick-off. A complete stroke passing across the four pick-offs therefore changes the value displayed by four units. Since this stroke can be made rapidly, a correction, even a large one, becomes very easy. The number of pick-offs is limited only by the necessity to have at least three if the direction of the finger stroke is to be detected.
Even though the use of capacitive pick-offs as fundamental means of control has been known for a long time, for example in elevators, televisions and radios, their application in watches is recent. That implies in the beginning that a pick-off should consist of an electrode sufficiently large (approximately the size of that portion of the finger which will activate it) to assure reliable operation and insensibility to industrial parasites. In addition, since this electrode should be mounted on the glass of the watch or on an insulated section of the front face of the watch case, the connection of the electrode to the circuit, which forms a unit with the module, poses difficult design problems.
In order to be able to use pick-offs with a small exposed surface, it is necessary to use sophisticated circuits, capable of reliably detecting the very small capacitance variations resulting from their activation. The Swiss Pat. No. 607872 from The Electronic Horological Centre describes such a circuit in which use is made of the synchronous detection of a voltage applied to the pick-off. The amplitude of the voltage detected varies depending upon whether the pick-off is activated or not. The risk of a false signal in this circuit, due to noise from industrial parasites of 50 and 100 Hz, is diminished as the frequency of the voltage applied to the pick-off is increased. With an applied frequency of approximately 8 kHz and careful fabrication, the functional reliability can be good. The application of the 8 kHz voltage to the pick-off requires, however, additional current, in the neighborhood of the current normally consumed by the circuit of a liquid crystal digital watch.